Gas detection instruments often use a pneumatic pump to draw a gas sample to the instrument from a remote location. Such pumps are used, for example, to sample the environment in a confined space (such as a manhole or a hold of a ship) before entry into the confined space. Pneumatic pumps also allow use of an extending sample probe to search for leaks along a gas line or for gas accumulations on a floor or a ceiling.
Most portable gas detection instruments run on batteries. If the pump motor is powered directly from the battery, the pump speed (and the flow rate) will decrease as the battery discharges. For this reason, motors are typically chosen to run at a voltage lower than the lowest anticipated battery voltage, and circuitry is added to maintain this voltage constant. To maximize battery life, an efficient method of driving the motor at the lower voltage must be employed. It is also desirable to run the pump at nearly a constant flow rate to minimize variations in the output of the one or more sensors of the gas detection instrument since sensor output may vary with flow rate.
To ensure proper operation, gas detection instruments incorporating pneumatic pumps typically require a device/method to control flow rate and to detect blocked flow or unacceptable flow rate decreases. In the pumping system disclosed in U.S. Pat. No. 5,295,790, a flow meter is used to directly measure volumetric flow rate through the pump and to provide feedback to a motor control circuit such that flow is controlled with accuracy regardless of variations in pump characteristics. Although feedback of a direct measurement of volumetric flow rate is an excellent method of pump control and fault detection in a gas detection instrument, it often requires a significant increase in manufacturing cost.
Manufacturing cost can be somewhat decreased through the use of simple volumetric flow meters such as rotometers for the detection of flow faults, but the performance of such rotometers is sensitive to the positioning thereof. Moreover, rotometers do not automatically activate an electronic alarm system and thus require constant operator observation.
Many portable gas detection instruments with pneumatic pumps use some form of electronic flow control/fault detection mechanisms based on an "indirect" or "inferential" measurement of flow rate. For example, a number of such instruments use hot wire anemometers or mass flow sensors to measure mass flow. These instruments, however, suffer from high power requirements, large size and high manufacturing costs.
Another common "indirect" method to detect blocked flow is the measurement of inlet suction at the pump. In general, a vacuum switch is used to produce an electrical signal when the suction exceeds a preset limit. While satisfactory as a detection scheme, the vacuum switches available for use in small, portable gas detection instruments have proven to be expensive and prone to mechanical or electrical failure in long term use.
Given the above-discussed and other drawbacks associated with current systems and methods for flow control and fault detection, it is very desirable to develop efficient and cost effective systems and methods for controlling and for detecting faults in pumping systems used in gas detection devices.